Altcoins tend to grow in a disproportionate way in the last stages of a Bitcoin bull market.
Would that be because of the misplaced over-confidence by traders that the bull market will go on longer than they thought?
I also interpret it that way. Some traders may think it's only a "dip" in Bitcoin's otherwise bullish performance and try to "hedge", then they see that - caused by this effect - some altcoins temporarily perform much better than Bitcoin, and the domino effect comes in ...
In contrast, in the latter half of the bear market, altcoins tend to crash heavily and lose almost all of their value. Many of them never see their position in the "market cap rankings" again, because in the next cycle usually new altcoins become the "divas" of the moment.
Would that be because of "capitulation" from altcoins? Most of the altcoins cannot be traded to fiat, but only with Bitcoin.
Yes, capitulation and the "new coinz of the block" effect

I believe the real reason are the poor offerings (and perspectives) of the overwhelming majority of altcoins. Most altcoins do not really solve a problem or offer a "killer feature", but are hyped driven by all sorts of promises, and their price is only driven by speculation, not by actual usage. These "hype-coins" disappear after a cycle after lots of traders lose their money, or they become buried by the next hypecoin wave. And I'm not even talking about ICO
tokens which are (often) far worse ...
XRP, as much as I consider it a centralized coin with a concept that doesn't really make sense, seems to have established itself in the top altcoin positions, just like Ethereum - it's one of the few coins never having fallen out of the Coinmarketcap top 10, since 2013. I think this is related to its banking-oriented ecosystem of users.
What users? Hahaha.
A German bank, for example, used it for cheap remittances to the US. Ripple is one of the few altcoins that
see actually some usage, although the big transaction amount is mostly related to irrelevant exchange activities.